How to overide custom node operator variables in the execution
self.node is set in invoke.
THe operator is designed to be invoked by pressing its button displayed on its node.
The operator defines
self.node = context.node

in its invoke method, and uses self.node in its execute method, and will hence not work unless invoked.
bpy.ops.foo.bar(override, 'INVOKE_DEFAULT', filepath=somefilepath)

IMO the simple fix is to edit the operator to use context.node as the node in place of self.node by placing
self.node = context.node

in the execute method also.
Alternatively return self.execute(context) from the invoke method if the value passed to self.filepath is a valid filepath and use
bpy.ops.foo.bar(override, 'INVOKE_DEFAULT', filepath=somefilepath)

to ensure it is invoked and node is set.
A note on overriding.
What you are passing in the override dictionary are context members, eg
override = {&quot;foo&quot; : &quot;bar&quot;}

then if passed as an override to  the operator
context.foo == &quot;bar&quot;

is true within the operator.
The filepath is a property of the operator, not the context and is hence not going to do diddly squat if passed as part of the override.  The operator (self) and context are not the same.  Pass it to the operator as a keyword variable.
Fork it.
Finally, fork and make the changes, or suggest / request  changes to the maintainer of the addon, so that it can be executed directly, or to find out if there are reasons why it cannot.